The structures and manufacturing processes for electronic packages are described in, for example, Donald T. Seraphin, Ronald Lasky, and Che-Yo Li, Principles of Electronic Packaging, McGraw-Hill Book Co., New York, N.Y. (1988) and Rao R. Tummala and Eugene J. Rymaszewski, Microelectronic Packaging Handbook, Van Nostrand Rinehold, New York, N.Y. (1988), disclosures of which are incorporated herein by reference.
An electronic circuit may contain many electronic circuit components, e.g. thousands or even millions of individual resistors, capacitors, inductors, diodes and transistors. These individual circuit components must be interconnected to form the circuits, and the individual circuits must be interconnected to form functional units. Power and signal distribution are done through these interconnections. The individual functional units require mechanical support and structural protection. The electrical circuits require electrical energy to function, and the removal of thermal energy to remain functional. Microelectronic packages, for example, chips, modules, circuit cards and circuit boards, are used to protect, house, cool and interconnect circuit components and circuits.
Within an integrated circuit, circuit component to circuit component and circuit to circuit interconnection, heat dissipation, and mechanical protection are provided by an integrated circuit chip. This chip enclosed within its module is referred to as the first level of packaging.
There is at least one further level of packaging. This second level of packaging is the circuit card. The circuit card is necessary for at least four functions. First, the circuit card is employed because the total required circuit or bit count to perform a desired function exceeds the bit count of the first level package, i.e. the chip. Second, the second level package, i.e. the circuit card, provides a site for components that are not readily integrated into the first level package, i.e. the chip or module. These components include capacitors, precision resistors, inductors, electromechanical switches, optical couplers, and the like. Third, the circuit card provides for signal interconnection with other circuit elements. Fourth, the second level package provides for thermal management, i.e. heat dissipation.
In most applications, there is a third level of packaging. This is the board level package. The board contains connector to accept a plurality of cards, and provide communication between the cards.
Increasing logic densities accompanied by increasing circuit densities impose higher thermal loads on electronic packages. In addition, power design, that is, card and board design, have been driven by the necessity of accommodating the ever-increasing density of logic or memory, with their concomitant increase in interconnections, in a smaller area. These higher density cards and boards have high power density, and therefore require sophisticated thermal management. In fact, the power demands have resulted in various power conversion cards, i.e. power supplies, of such design that the operating temperatures of the power conversion cards are usually at about 90.degree.-110.degree. C.
However, such relatively high card temperatures exceed the typically acceptable operating temperatures of about 75.degree. C. and less for memory and logic chips as well as for capacitors and certain resistors. Accordingly, power supply functions and active functions such as logic and/or memory functions are typically placed onto separate cards in order to provide a reliable system.
A typical power supply card of interest is constructed of a top signal/component mounting layer and a bottom ground/heatsink layer separated by a thin, thermally enhanced dielectric material. The overall size of the carrier may range from at least about 1" in length or width, and generally about 2" by 4" dimensions. The top signal layer may be of any conducting material, generally about 0.001" to about 0.005" thick copper, and preferably about 0.003" to about 0.004". The defined signal layer contains pads for mounting of discrete devices including resistors, capacitors, inductors, transformers, integrated circuits and the like. The signal layer also includes circuit tracks for electrical interconnection of the various devices. The signal layer also functions to quickly spread heat from high power dissipation devices such as back side solder bonded, wire bonded power integrated circuits, i.e. heat spreader. The dielectric material is designed to efficiently transfer heat from the signal layer to the heatsink layer, while still providing electrical insulation between the two layers. The ground/heatsink layer is generally a thick metal plate, and preferably about 0.020" to about 0.1080" thick copper.
The thermal dissipation path from a back bonded chip on this type of carrier includes solder layer of about 0.002" to about 0.005" thick which transfers heat from the chip back side into the carrier signal layer. Heat is then spread in the horizontal direction through the signal layer heat spreader, followed by transfer through the dielectric layer into the back side heatsink layer where further horizontal spreading and transfer to ambient occurs. The thermal dissipation path can be further enhanced by one of several methods including copper plated or solder filled through holes or blink vias from the signal layer heat spreader to back side heat sink, or direct attach of the power chip to the heat sink through a window in the dielectric where the back side surface has been coined up through the top surface of the carrier, and the dielectric and top signal layers milled off. Enhancement of heat transfer from the heatsink to ambient can be achieved by etched grooves in the heatsink that increase surface area for radiative transfer and channel air flow for convective cooling.
More particularly, power supply carriers have several unique features that distinguish them from common circuit boards. For instant, power supplies contain several components (diodes and power chips) that require the dissipation of significant amounts of energy in order to maintain an acceptable chip junction temperature. Energy dissipation is currently managed by conducting the heat from the soldered component through a dielectric (with enhanced thermal conductivity) to a heat dissipating back plane. Such a configuration is so efficient in heat dissipation that the entire carrier is maintained at a uniformly high temperature. However, as mentioned above, the integration of the power functions with the active functions such as logic and/or memory functions is not practical or possible because the active functions such as the logic and/or memory functions cannot operate reliably at temperatures greater than about 75.degree. C.